I'd been playing a game of Doom episode 2 and had just started map E2M4. I ran forwards and started shooting the bad guys at the far end of the start hallway and a message popped up. Unfortunately, I thought I was recoding a log file so I didn't bother memorising/writing down the message but it turns out, I'd typed the logfile command incorrectly and wasn't recording. I *think* the error message mentioned "r_column" and "access violation". Shortly after the message appeared, I started getting graphical glitches (see screenshots - resized from 1920x1200).

I quit, restarted and warped to E2M4 and there was no problem. I also loaded the autosave from the start of E2M4 and, again, no problem.

Just to be clear, the game didn't actually crash. Perhaps it might have if I'd left it for a while but I did actually play with the visual corruption (watching the effects) for a good few minutes the first time it happened without a crash happening.